Dimethylolpropionic acid based electrolytes for electrical capacitors

ABSTRACT

An electrolytic capacitor containing new electrolyte systems comprising a solvent containing a compound derived from Alpha , Alpha -dimethylolpropionic acid:

[451 Nov. 7, 1972 United States Patent I Ross et al.

[541 DIMETHYLOLPROPIONIC ACID 2,022,500 11/1935 Clark et .317/230 3,539,881 11/1970 Anderson...........;......317/230 3,580,845 5/1971 Dahle ........................317/235 BASED ELECTROLYTES FOR ELECTRICAL CAPACITORS [72] Inventors: Sidney D. Ross; Franz S. Dunkl,

Primary Examiner-James D. Kallam both of Wilhamstown; Manuel Finkelstein, North Adams, all of Mass. Attorney-Vincent Sweeney [73] Assignee: Sprague Electric Company, North [57] ABSTRACT Adams An velectrolytic capacitor containing new electrolyte [22] Filed: Dec. 3, 1971, systems comprising a solvent containing a compound I2" App No 204 1 derived from a, a-dimethylolpropionic acid:

CHzOH 0113-15-0 0 on [52] US. [51] Int. Cl. 9/00 .317/230, 231, 233; 252/622 [58] Field of Search [56] 9 Claims, 1 Drawing Figure References Cited UNITED STATES PATENTS 2,036,669 Yngve .......................317/230 1 DIMETHYLOLPROPIONIC ACID'BASED ELECTROLYTES FOR ELECTRICAL CAPACITORS BACKGROUND OF THE INVENTION trolytes for'electrolytic capacitors. Of this multitude of electrolytes which have been disclosed, few have achieved commercial significance. At present, most electrolytic capacitors employ either relatively simple borate and water systems, or .else rather complex borate systems which contain anion species other than the simple boratein a suitable organic solvent. Many prior art electrolytes, e.g. ammonium pentaborate in ethylene glycol, dissolve anodic aluminum oxide to a considerable degree upon heating at or near 85 C.

It is therefore an object of the present invention to provide a relatively simple, alternative electrolyte system for foil capacitors, capable of :being substituted for ones presently in use.

It is another object of the present invention to provide a capacitor employing-an electrolyte which does not attack anodicaluminum oxide at elevated temperatures.

It is a further object of this inventionlto provide an electrolytic capacitor capable of long term reliable operation under temperature conditions varying from -55C up to 125C.

SUMMAR'YIOF THE'INVENTION Electrolyte systems have been prepared from a a di-m'ethylolpropionic acid,

cmon

om- COOH so as to provide electrolytic capacitors with electrolytes that are reliable under a wide variety of operating temperatures and voltages.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE of the drawing is an isometric view of a partially unwound capacitor of the present invention. The capacitor electrodes contemplated herein may be tantalum or aluminum. H

The drawing illustrates a capacitance section of the type with which the present invention is concerned. Anode ll of the capacitance section 10 is of a valve metal such as tantalum or aluminum having formed on its surface an oxide layer which functions as the dielectric. Cathodic contact 12 is of a valve metal or inert metal, as desired. Porous spacers l3 and 14 are fully impregnated with the electrolyte compositions of this invention. Tabs 15 and 16 are connected to the electrode foil 11 and cathodic contact foil 12 respectively as terminals. The capacitance section 10 is then placed in a capacitor housing (not shown) that can be a metal or non-metal container.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, this invention provides an electrolytic capacitor containing an electrolyte based on a solvent having dissolved therein a compound based on a a dimethylolpropionic acid. The following compounds can be used with dimethylformamide or other solvents as electrolytes of this invention, but are not to be construed as limiting.

EXAMPLE I Tetramethylammonium a a -dimethylolpropionate,

CHIOH (CHa)tN*CH:- 000" per 0.l mole ofthe salt); m.p. 218 C.

EXAMPLE II Tetramethylammonium boro-dia a -dimethylol- .propionate. This was prepared by dissolving a a dimethylolpropionic acid (2 'moles), adding, first, tetramethylammonium hydroxide (1 mole) as a 10% aqueous solution and then, boric acid (1 mole) dissolved in water. The solution is taken to dryness with the water pump and crystallized from isopropanolether. Most of this product was soluble in isopropanol, and the product from this solvent had a melting point in the range of l99-204 C.

Analysis calculated for C H BNO C, 48.16; H, 8.08; N, 4.01. Found: C, 48.46; H, 8.39; N, 4.63.

The isopropanol insoluble material was crystallized from methanol-ether and had a melting point in the range of 205-209C.

Analysis calculated for C H BNO C, 48.16; H, 8.08; N, 4.0l.'Found: C, 47.36: H, 8.16; N, 3.82.

When used with dimethylformamide as an electrolyte, the two compounds that were isolated appear to be electrically equivalent.

EXAMPLE Ill Diisopropylammonium a a -dimethylolpropionate,

This-salt was prepared by neutralizing a methanolic solution of the acid with a methanolic solution of the amine, removing the solvent and crystallizing from methanol-ether; m.p. 98l00 C.

EXAMPLE IV I 1,4-diazabicylooctane a a -dimethylolpropionate,-

TABLE 1 Maximum Conc H 80, Resistivity Formation Salt mole" mole ohm-cm Voltage Example l 0.1 I 305 260 l 0.25 0 169 202 11 0.1 0 372 180 ll 0.3 0 207 153 ll 0.] 0.2 411 203 1 111 0.1 0 2070 408 111' 0.25 0 1130 348 111 0.25 0.125 530 415 111 0.25 0.25 461 425 IV 0.30 0 1250 327 IV 0.30 0.15 488 270 The boron complexes formed'in Examples 111 and IV by the addition of boric acid thereto would be diisopropylammonium boro-di-a a -dimethylolpropionate and 1,4-diazabicylooctane boro-di-a a dimethylolpropionate.

The diisopropylammonium a a -dimethylolpropionate can be prepared in situ in dimethylformamide by adding appropriate quantities of the amine and the acid to the solvent. The results with electrolytes prepared this way are shown in Table 11, wherein A is the concentration of diisopropylamine, B is the concentration of a a -dimethylolpropionic acid, and C is the concentration of added boric acid. The boric acid additions result in the formation of complexes having the same anions as those of Example 11, but with the associated cation being the diisopropylammonium ion.

Electrolytic capacitors prepared with the following electrolyte systems gave satisfactory aging and satisfactory to excellent shelf life characteristics. Some of these include the following:

1. 0.5M tetramethylammonium a a -dimethy1olpropionate in dimethylformamide (DMF) prepared by mixing 200 g. DMF, 23.2 g. of this salt and 4.5 ml. water, dissolving and filtering. The resistivity of this electrolyte was 119 ohm-cm. The capacitance sections impregnated with theelectrolyte possessed the rating of 1.1.11, 60 volts DC. The following values given are averages of 5 units during a test at C with 50 volts DC applied.

Hours con RC(ohm-1Lf) l(ma) Initial 1 16.0 27 2.3 500 115.8 40 0.37 1000 1 14.8 48 0.35

The foregoing data indicate that the capacitors are electrically stable and the leakage current values are. good. Also this data indicates that there is no attack of theanodic aluminum oxide by the solvent thereof, at this temperature.

2. 0.1M tetramethylammonium a a -dimethylolpropionate in dimethylformamide. The, resistivity of this electrolyte was 305 ohm-cm. The capacitance sections impregnated with the electrolyte possessed the rating of 60 pf, 50 volts DC. The following values given are averages of 5 units during'a test at 125 C. with 50 volts DC applied.

Hours C(fl-f) RC(0hm-p.f) 1( ma) initial 68.92 63 3.4

RC C

C C C RC As can be observed, very little change has occured in the capacitance at 25 C after going through the above temperature cycle.

4. An electrolyte was prepared, in situ, in DMF using 3.35 g. dimethylolpropionic acid, 1.26 g. diisopropylamine, 2.32 g. boric acid and a small amount of salicylic acid added to enhance formation of the areas not covered by oxide, such as the cut edges of the anode. The resistivity of this electrolyte was 465 ohm-cm. The capacitance sections impregnated with the electrolyte possessed therating of 27 p.f., 250 volts DC.

5. An electrolyte was prepared, in situ, in DMF using 3.35 g. dimethylolpropionic acid, 2.32 g. tributylamine, 2.32 g. boric acid and a small amount of salicylic acid added thereto. The resistivity of this electrolyte was 590 ohm-cm. The capacitance sections impregnated herewith possessed the rating of 27 pf. 250 V.

The foregoing data indicate that the capacitors are electrically stable and the leakage current values are good. The compound formed thereby is tributylammonium boro-dia a -dimethylolpropionate.

The tetramethylammonium boro-dia a dimethylolpropionate was studied as an electrolyte solute in N-methylformamide and formamide, as well as in dimethylformamide. The first two solvents are of special interest, since, compared to di-methylformamide, N-methylformamide increases the porosity of the paper spacer fourfold and formamide increases it eight fold. The electrolyte systems contemplated herein are not limited to those wherein the solvent isdimethylformamide, N-methylformamide, or formamide since other solvents and mixtures may be employed. For example. water, ethylene cyanohydrin, the cellosolves, glycols, N-mcthylpyrrolidonc.dimethylsulfoxide, etc., and mixtures thereof may be employed. It is also to be understood that the electrolytes of the present invention have utility in capacitors other than aluminum capacitors, e.g. tantalum, niobium, etc.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not limited to. said details except as set forth in the appended claims.

' What is claimed is:

1. An electrolytic capacitor comprising a capacitor housing; a capacitance section within said housing having a plurality of electrodes, at least one electrode being a valve metal having on its surface an insulating oxide layer, said section being impregnated with an electrolyte consisting essentially of a solvent having dissolved therein a compound derived from a d dimethylolpropionic acid.

2. The capacitor of claim 1 wherein said compound is tetramethylammonium a a dimethylolpropionate.

3. The capacitor of claim 1 wherein said compound is tetramethylammonium boro-dia a -dimethylolpropionate.

4. The capacitor of claim 1 wherein said compound is diisopropylammonium a a -dimethylolpropionate.

5. The capacitor of claim Iwherein said compound is l,4-diazabicylooctane a a' -dimethylolpropionate.

6. The capacitor of claim 1 wherein said compound is diisopropylammonium boro-dia a -dimethylolpropionate.

7. The capacitor of claim 1 wherein said compound is l,4-diazabicylooctane boro-dia a -dimethylolpropionate.

8. The capacitor of claim 1 wherein said compound is tributylammonium boro-dia a -dimethylolpropionate.

9. The capacitor of claim 10 wherein said valve metal is aluminum, said oxide layer is aluminum oxide, said solvent is dimethylformamide, and said compound is tributylammonium boro-di a,a-dimethylolpropionate.

10. The capacitor of claim 1 wherein said solvent is at least one member selected from the group consisting essentially of dimethylformanide, N methylformamide, and formamide. 

2. The capacitor of claim 1 wherein said compound is tetramethylammonium Alpha , Alpha -dimethylolpropionate.
 3. The capacitor of claim 1 wherein said compound is tetramethylammonium boro-di- Alpha , Alpha -dimethylolpropionate.
 4. The capacitor of clAim 1 wherein said compound is diisopropylammonium Alpha , Alpha -dimethylolpropionate.
 5. The capacitor of claim 1 wherein said compound is 1,4-diazabicylooctane Alpha , Alpha -dimethylolpropionate.
 6. The capacitor of claim 1 wherein said compound is diisopropylammonium boro-di- Alpha , Alpha -dimethylolpropionate.
 7. The capacitor of claim 1 wherein said compound is 1,4-diazabicylooctane boro-di- Alpha , Alpha -dimethylolpropionate.
 8. The capacitor of claim 1 wherein said compound is tributylammonium boro-di- Alpha , Alpha -dimethylolpropionate.
 9. The capacitor of claim 1 wherein said valve metal is aluminum, said oxide layer is aluminum oxide, said solvent is dimethylformamide, and said compound is tributylammonium boro-di Alpha , Alpha -dimethylolpropionate. 